1. Field of the Invention
The present invention relates to a filter device which transforms the shape of an input signal and to an electronic musical instrument using such filter device, and more particularly to a digital filter device which changes the strength and volume of sounds, and further changes tone color of musical tones in response to playing operation of a performer, for example, in response to performance data varying with time supplied from a keyboard and to an electronic musical instrument employing such digital filter device.
2. Description of the Related Art
In recent, techniques of processing digital signals have been developed and a number of electronic musical instruments are now in use which employ digital filters for varying tone color of the musical tones. One example of such electronic musical instruments is disclosed in Tokkai-sho 59-44096, which uses a digital filter as a circuit for varying tone color of the musical tones. In such an electronic musical instrument, a set of filter parameters are supplied to the digital filter and thereby the filtering characteristics (frequency vs. amplitude characteristics) are set. This type of digital filter controls parameters such as a cut-off frequency fc and a resonance. Variations in these parameters accompany with variations in tone color of musical tones. The cut-off frequency fc of the filter has a physical meaning as follows. In general, sounds generated by a sound source contain a number of harmonics. When the sound signal is applied to the digital filter, frequency components contained therein higher than the cut-off frequency fc are abruptly reduced in amplitude.
When, for example, the cut-off frequency fc of a low pass filter is set comparatively low, high frequency components contained in the sound signal are reduced. As the result, the output signal of the low pass filter generates sweet and soft sounds. When the cut-off frequency fc of the low pass filter is set higher, the sound signal of a wider frequency spectrum, i.e., the sound signal of a frequency spectrum substantially equivalent to the original sound is allowed to pass the filter. The frequency vs. amplitude characteristics are shown in FIG. 30. The resonance has effects that abruptly increase amplitudes of frequency components of the sound signal in the neighborhood of the cut-off frequency fc. As the result, the resonance will add peculiar effects to the sound.
In the conventional electronic musical instrument employing the digital filter mentioned above, only the cut-off frequency fc of the filter is changed in response to the respective performance data supplied thereto. Therefore, only a predetermined range of frequency components contained in the sound signal are reduced, i.e., a predetermined range of harmonics contained in the sound signal are reduced only in their amplitudes; besides, the tone color of the musical tones is changed only in a simple manner. In most of acoustic instruments, not only a frequency range in which harmonics are reduced is changed but also other parameters are changed. Therefore, an exquisite expression in the musical performance is not fully obtained only by changing the cut-off frequency fc of the filter. Since an attenuation factor in the cut-off frequency range of the filter is not variable and the resultant performance data is constant, electronic musical instruments are required to change another factors to give a wide variety of expressions in the musical performance.